The present disclosure relates to a cellular confinement system, also known as a CCS or a geocell, which is suitable for use in supporting loads, such as those present on roads, railways, parking areas, and pavements. In particular, the geocells of the present disclosure retain their dimensions after large numbers of load cycles and temperature cycles; thus the required confinement of the infill is retained throughout the design life cycle of the geocell.
A cellular confinement system (CCS) is an array of containment cells resembling a “honeycomb” structure that is filled with granular infill, which can be cohesionless soil, sand, gravel, ballast, crushed stone, or any other type of granular aggregate. Also known as geocells, CCSs are mainly used in civil engineering applications that require little mechanical strength and stiffness, such as slope protection (to prevent erosion) or providing lateral support for slopes.
CCSs differ from other geosynthetics such as geogrids or geotextiles in that geogrids/geotextiles are flat (i.e., two-dimensional) and used as planar reinforcement. Geogrids/geotextiles provide confinement only for very limited vertical distances (usually 1-2 times the average size of the granular material) and are limited to granular materials having an average size of greater than about 20 mm. This limits the use of such two-dimensional geosynthetics to relatively expensive granular materials (ballast, crushed stone and gravel) because they provide hardly any confinement or reinforcement to lower quality granular materials, such as recycled asphalt, crushed concrete, fly ash and quarry waste. In contrast, CCSs are three-dimensional structures that provide confinement in all directions (i.e. along the entire cross-section of each cell). Moreover, the multi-cell geometry provides passive resistance that increases the bearing capacity. Unlike two-dimensional geosynthetics, a geocell provides confinement and reinforcement to granular materials having an average particle size less than about 20 mm, and in some cases materials having an average particle size of about 10 mm or less.
Geocells are manufactured by some companies worldwide, including Presto. Presto's geocells, as well as those of most of its imitators, are made of polyethylene (PE). The polyethylene (PE) can be high density polyethylene (HDPE) or medium density polyethylene (MDPE). The term “HDPE” refers hereinafter to a polyethylene characterized by density of greater than 0.940 g/cm3. The term medium density polyethylene (MDPE) refers to a polyethylene characterized by density of greater than 0.925 g/cm3 to 0.940 g/cm3. The term low density polyethylene (LDPE) refers to a polyethylene characterized by density of 0.91 to 0.925 g/cm3.
Geocells made from HDPE and MDPE are either smooth or texturized. Texturized geocells are most common in the market, since the texture may provide some additional friction of the geocell walls with the infill. Although HDPE theoretically can have a tensile strength (tensile stress at yield or at break) of greater than 15 megapascals (MPa), in practice, when a sample is taken from a geocell wall and tested according to ASTM D638, the strength is insufficient for load support applications, such as roads and railways, and even at a high strain rate of 150%/minute, will barely reach 14 MPa.
The poor properties of HDPE and MDPE are clearly visible when analyzed by Dynamic Mechanical Analysis (DMA) according to ASTM D4065: the storage modulus at 23° C. is lower than about 400 MPa. The storage modulus deteriorates dramatically as temperature increases, and goes below useful levels at temperatures of about 75° C., thus limiting the usage as load support reinforcements. These moderate mechanical properties are sufficient for slope protection, but not for long term load support applications that are designed for service of more than five years.
Another method for predicting the long term, creep-related behavior of polymers is the accelerated creep test by stepped isothermal method (SIM) according to ASTM 6992. In this method, a polymeric specimen is subjected to constant load under a stepped temperature program. The elevated temperature steps accelerate creep. The method enables extrapolation of the specimen's properties over long periods of time, even over 100 years. Usually, when PE and PP are tested, the load that causes plastic deformation of 10% is called the “long term design strength” and is used in geosynthetics as the allowed strength for designs. Loads that cause plastic deformation greater than 10% are avoided, because PE and PP are subject to second order creep above 10% plastic deformation. Second order creep is unpredictable and PE and PP have a tendency to “craze” in this mode.
For applications such as roads, railroads and heavily loaded storage and parking yards, this strength of barely 14 MPa is insufficient. In particular, geocells with these moderate mechanical properties tend to have relatively low stiffness and tend to deform plastically at strains as low as 8%. The plastic deformation causes the cell to lose its confining potential, essentially the major reinforcement mechanism, after short periods of time or low numbers of vehicles passing (low number of cyclic loads). For example, when a strip taken from a typical geocell in the machine direction (perpendicular to seam plane) is tested according to ASTM D638 at a strain rate of 20%/minute or even at 150%/minute, the stress at 6% strain is less than 13 MPa, at 8% strain is less than 13.5 MPa, and at 12% strain is less than 14 MPa. As a result, HDPE geocells are limited to applications where the geocell is under low load and where confinement of load-bearing infill is not mandatory (e.g. in soil stabilization). Geocells are not widely accepted in load support applications, such as roads, railways, parking areas, or heavy container storage areas, due to the high tendency of plastic deformation at low strains.
When a vertical load is applied to a substrate of a granular material, a portion of that vertical load is translated to a horizontal load or pressure. The magnitude of the horizontal load is equal to the vertical load multiplied by the coefficient of horizontal earth pressure (also known as lateral earth pressure coefficient or LEPC) of the granular material. The LEPC can vary from about 0.2 for good materials like gravel and crushed stone (generally hard particles, poorly graded, so compaction is very good and plasticity is minimal) to about 0.3 to 0.4 for more plastic materials like quarry waste or recycled asphalt (materials that have a high fines content and high plasticity). When the granular material is wet (e.g. rain or flood saturating the base course and sub-base of a road), its plasticity increases, and higher horizontal loads are developed, providing increased hoop stress in the cell wall.
When the granular material is confined by a geocell, and a vertical load is applied from the top by a static or dynamic stress (such as pressure provided by a vehicle wheel or train rail), the horizontal pressure is translated to hoop stress in the geocell wall. The hoop stress is proportional to the horizontal pressure and to the average cell radius, and is inversely proportional to the thickness of the cell wall.
  HS  =            VP      *      LEPC      *      r        d  wherein HS is the average hoop stress in the geocell wall, VP is the vertical pressure applied externally on the granular material by a load, LEPC is the lateral earth pressure coefficient, r is the average cell radius and d is the nominal cell wall thickness.
For example, a geocell made of HDPE or MDPE having a cell wall thickness of 1.5 millimeters (including texture, and the term “wall thickness” referring hereinafter to the distance from peak to peak on the cell wall cross-section), an average diameter (when infilled with granular material) of 230 millimeters, a height of 200 millimeters, filled with sand or quarry waste (a LEPC of 0.3), and a vertical load of 700 kilopascal (kPa), would experience a hoop stress of about 16 megapascals (MPa). As seen from the hoop stress equation, larger diameter or thinner walls—which are favored from a manufacturing economy point of view—are subjected to significantly higher hoop stresses, and thus do not operate well as reinforcement when made of HDPE or MDPE.
Vertical loads of 550 kPa are common for unpaved roads. Significantly higher loads, of 700 kPa or more, may be experienced in roads (paved and unpaved) for heavy trucks, industrial service roads, or parking areas.
Because load support applications, especially roads and railways, are generally subjected to millions of cyclic loads, the geocell wall needs to retain its original dimensions under cyclic loading with very low plastic deformation. Commercial usage of HDPE geocells is limited to non load-bearing applications because HDPE typically reaches its plastic limit at about 8% strain, and at stresses below typical stresses commonly found in load support applications.
It would be desirable to provide a geocell that has increased stiffness and strength, lower tendency to deform at elevated temperatures, better retention of its elasticity at temperatures above ambient (23° C.), reduced tendency to undergo plastic deformation under repeated and continuous loadings, and/or long service periods.